1. Field of the Invention
The present invention relates to a multilayer capacitor, and more particularly, to an improvement to facilitate control of an equivalent series resistance (ESR) of the multilayer capacitor.
2. Description of Related Art
If voltage in a power supply line varies significantly due to an impedance between the power supply line and a ground, the voltage variation may cause the operation of a driven circuit to be unstable, cause inter-circuit interference via the power supply circuit, and generate an oscillation. To avoid these problems, a decoupling capacitor is usually connected between the power supply line and the ground. The decoupling capacitor reduces the impedance between the power supply line and the ground, and suppresses variations of the power supply voltage and the inter-circuit interference.
Recently, in communication equipment, such as cell-phones, for example, and information processing equipment, such as personal computers, for example, there is a trend toward a higher signal rate and a higher clock frequency of ICs used therein to process a greater amount of information. Therefore, noise including greater amounts of higher harmonic components is more likely to occur, and an IC power supply circuit requires stronger decoupling.
To increase the decoupling effect, a decoupling capacitor with a superior impedance-frequency characteristic may be used. One example of such a decoupling capacitor is a multilayer ceramic capacitor. The multilayer ceramic capacitor has a smaller ESL (equivalent series inductance) and has a higher noise absorption effect over a wider frequency band than an electrolytic capacitor.
Another role of the decoupling capacitor is to supply charges to an IC. Usually, the decoupling capacitor is disposed near the IC. When a voltage variation occurs in the power supply line, charges are quickly supplied to the IC from the decoupling capacitor so as to prevent a delay in the supply of the charges to the IC.
During a charge or a discharge to or from a capacitor, a counter electromotive force dV expressed by a formula of dV=L·di/dt is generated. If dV has a large value, supply of the charge to the IC is delayed. With an increasing demand for a higher clock frequency of the IC, a current variation di/dt per unit time tends to increase. Thus, the inductance L must be reduced in order to reduce dV. For that reason, the ELS of a capacitor must be further reduced.
As a low-ESL multilayer ceramic capacitor having a reduced ESL, an LW-reversed multilayer ceramic capacitor is known, for example. In a conventional multilayer ceramic capacitor, a length of an end surface of a capacitor body on which an external terminal electrode is provided (dimension W) is less than a length of a side surface of the capacitor body adjacent to the end surface (dimension L). On the other hand, in the LW-reversed multilayer ceramic capacitor, a length of an end surface of a capacitor body on which an external terminal electrode is provided (dimension W) is greater than a length of a side surface thereof (dimension L). In the LW-reversed multilayer ceramic capacitor with these dimensions, the ESL is reduced because current paths inside the capacitor body are wider and shorter.
In this type of low-ESL multilayer ceramic capacitor, however, because of the wider and shorter current paths as described above, the ESR is also reduced.
Furthermore, the multilayer ceramic capacitor must have a greater capacity. The capacity of the multilayer ceramic capacitor can be increased by increasing the number of ceramic layers and the number of internal electrodes. In such a case, however, the ESR is reduced because the number of current paths is increased. In other words, with the increasing demand for a lower ESL and a greater capacity, the ESR of the multilayer ceramic capacitor tends to be further reduced.
It is, however, known that if the ESR of the capacitor becomes too low, an impedance mismatch is caused in a circuit, and a damped oscillation, called “ringing”, to distort the rising of a signal waveform is more likely to occur. The occurrence of the ringing may cause a malfunction of the IC due to a distorted signal.
In addition, if the ESR of the capacitor becomes too low, an impedance-frequency characteristic of the capacitor becomes too steep near the resonance frequency. Accordingly, the capacitor will have a high antiresonance point against the resonance frequency of another capacitor mounted nearby, thus resulting in a risk that the noise absorption effect in a frequency band near the antiresonance point may be reduced.
To prevent the above-described undesired phenomenon, it is effective to connect resistance elements to a line in series so as to reduce the sharpness of the impedance-frequency characteristic curve. Recently, a resistance component has been provided in the capacitor itself to control the ESR of a capacitor.
For example, Japanese Unexamined Patent Application Publication No. 2004-47983 and International Publication No. 2006/022258 disclose techniques to control the ESR by providing a resistance component in an external terminal electrode that is electrically connected to an internal electrode. In particular, International Publication No. 2006/022258 discloses a multilayer ceramic capacitor in which an external terminal electrode including a resistance component is formed by the steps of dipping a capacitor body in a resistance paste which includes a resistance material, e.g., ITO, and firing the resistance paste coated on the capacitor body.
When the resistance component is included in the external terminal electrode as described in Japanese Unexamined Patent Application Publication No. 2004-47983 and International Publication No. 2006/022258, the ESR of the capacitor can be controlled, for example, by adjusting the specific resistance of the resistance material or the coating thickness of the resistance paste.
However, preparing several types of resistance pastes to adjust the specific resistance of the resistance material is cumbersome and expensive. In addition, there is a risk that a change in the composition of the resistance paste to adjust the specific resistance may affect other factors, such as the reactivity with the internal electrode and the adhesion to the capacitor body.
Furthermore, the viscosity of the resistance paste needs to be adjusted in order to adjust the coating thickness of the resistance paste. In this case, the composition of the resistance paste must be changed, which may also affect other factors. Another problem is that the amount that the thickness of the resistance paste can be increased is limited, thus increasing the technical restrictions on the control of the ESR, particularly, in increasing the ESR.